Isomerization of xylenes



2 Sheets-Sheet 1 c. s. BENNETT ET AL ISOMERIZATI ON OF XYLENES Aug. y14, 1951 Filed Jan. 9, 195o Aug. 14, 1951 y 2 sheets-snee. 2

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1.1mm m u. d l. .D Du? Patented Aug. 14, 195.1.-

Clarenoellennett,

ImBeachmdWilliamA.

'* Bauey,Ji-..wnnntcnek,csuf..mnmto Shell Development Company, San Francisco, Calif., a corporation o! Delaware l Application January i, 1050, Suhl No. 137,592

This invention relates to the catalytic isomerization of xylenes. e

It is well known that the -isomeric xylene, namely o-sylene, m-xylene, and p-xyle'ne. can be converted one into the other by isomerizetion. However, the isomerization is ditlicult to accomplish. When the isomerization is carried out under conditions to give a relatively selective and clean-cut isomerization the reaction takes place at a rate which is too slowfor practical. commercial operation. Even when the isomerization is carried out under relatively drastic conditions leading -to appreciable degradation, the reaction is quite slow. 'I'he xylenes may be isomerized by purely thermal means or by treatment under certain conditions with Friedel-Crafts time catalysts, e. g. aluminum chloride plus hydrogen chloride, but the required conditions are severe, the reaction is' v ery slow, and the yields are poor. The xylenes may also be isomerized at temperatures in the order of 7501000 F. through the use oi.' a silica-alumina type catalyst as shown, for instance, in- U. S. Patent No. 2,403,757. When operating under conditions to avoid extensive cracking and disproportionation the isomerization is quite slow, and when operating under more severe conditions the predominant reactions are cracking and disproportionation and the yields are.

very low (see Ind. Eng. Chem. 37, 671 (1945))..

In the process of this invention i'tylenes are iso, merized at an enhanced rate by treating them with hydrogen and/or steam under suitable conditions of temperature and pressure in the presence of a carbonium ion-forming catalyst. In a preferred embodiment of the invention the isomerization is also carried out in the presence of added ethyl benzene.

As is lmown,4 the xylenes tend to approach an I. equilibrium composition when treated under iso- 800 F. 0 F. 1160" F.

o-xylene, mole per cent.. 22. 4 22. 8 23. 1 m-xylene, mole per cent 47. 8 45. 8 43. 9 xylene. mole per cent 2l. 6 m. 6 19. 8 thylbenzone, mola per cant-------- 8. 3 l0. 8 18. 2

formsr of in the present processrfrom any other composition of the respective isomers. Thus, for example, o-xylene and/or m-xylene may be converted to P-xylene, o-xylene and/or p-xylene may be converted to m-xylene, or m-xylene and/or p-xylene may be converted to o-xylene. While ethylbenaene must be considered in the equilibrium composition, its action is dierent than that of the xylenes in the isomerization and its eiIect requires separate consideration. While we may isomerize any pure xylene or mixture oi xylenes having other than the equilibrium composition. we prefer to isomerize either a relatively pure xylene or a mixture appreciably removed from the equilibrium composition. The 'process is particularly suited, for example, for the production oi o-xylene and p-xylene from commercial m-xylene concentrates. Usually one has as a starting material the usual commercial xylene which consists of a mixture of the various isomers. The composition of such mixtures usually is not too :far removed from the equilibrium composition. In this usual case it is desirable to remove a part of the o-xylene, e. g., by superfractionation. A part of the D-Xylene may then be separated in a substantially pure form by crystallization if desired. 'Ihe resulting mixture rich in m-xylene may then be advantageously isomerized to produce further quantities of o-xylene and/o1` p-xylene.

In the method of this invention the xylene is isomerized through the use of a carbonium ionforming catalyst. It is found that certain catalysts are operative through their ability to catalyze the formation oi carbonium ions and that catalysts of this type are required in the present process. The group of catalysts of various cornpositions known in the art as clay type cracking catalysts are of this type and are the preferred catalysts for use in the present process. Although these .solid microporous clay type catalysts may be made from various other materials, the most common catalysts of the type are either special 'acid treated clays or synthetic materials composed largely of silica in combination with alu' mina, magnesia, zirconia or boric oxide. Since these catalysts and their preparation are well known in the art, and since the use of a, catalyst of this particular type is not considered to be a unique feature of the process, it is not considered necessary to discuss them in further detail.

The isomerization may be carried out at any pressure. Pressures from essentially atmospheric pressure up to above 500 atmospheres have been applied with substantially the same results. Temperatures within the range of about 800 F. to about 1100" F. are applicable. 'lfhe space rate 3 is correlated with the temperature in the usual manner to afford the desired degree of conversion and may vary from about 1 to about 16. The space rate is specied in terms of the liquid hourly space velocity (L. H. S. V.) which is defined as the volumes of the hydrocarbon feed, measured as a liquid, passed in contact with the volume of the catalyst'in one hour.

The most important feature of the present method is based upon the discovery that, for some reason which cannot be explained, the isomerization using the speciiled particular type of catalyst is considerably enhanced by the presence of appreciable amounts of hydrogen and/or steam. Steam is more effective than hydrogen. The effects of steam and hydrogen in materially increasing the rate of isomerization under these conditions will be seen in the following examples in which a typical commercial feed having the following inspection data was used.

Ortho-lean Xylene Feed Gravity, API 3l. 8 Aromatics, by sulionation, per cent v 9s ASTM Distillation, F.:

IBP- 271 FBP 312 10o/7 275 in 277 90 278 as 279 Precision Distillation, per cent v.:

200 0. 0 250 0. 0 300 96. 7 300+F 3. 3 AnaiysisLper cent v.:

Tnlnena 0. 2 Ethylhan nnn 8 Q o-Xylene 5. 0 mXylene 60. 1 p-Xylene 25. 8

This feed was a typical commercial r11-xylene concentrate produced by the removal of a part of the o-xylene by superfractionation.

The isomerizations were carried out under various conditions ranging between the following limits:

Temperature 305 F.ll0l F, Pressure 1 atm-57 atm. L. H. S. V 1-16 with and without various amounts of various added materials. The enhancement in the isomerization by steam and hydrogen is illustrated by the following conversions obtained in comparable operations.

Conversion per cent of $0922 Steam or hydrogen, moles/mole of feed o-xylene b v* Cs Egl'llb aromatlcs N nnn 80. 9 87. 3 Hydrogen, 0.2:1 85. 4 90. 0 Hydrogen, 0.6:1 88. l 91.6 Steam, 0.2:1..... 85. 9 86. 4 Steam, 0.6:1-- 89. 8 89. 7

a,t..4,sss

be seen that both steam and hydrogen are quite effective in enhancing the isomeriation. Since, as pointed out, the rate declines rapidly as the equilibrium composition is approached, it is considered that the diiferences in conversion shown in the range oi Sil-% of the equilibrium concentration of o-xylene are quite significant.

While the improvement shown in graph I is quite signicant the differences in the conversion in this range are even better illustrated when based on a common recovery. Thus, when the conversions are based (through suitable correlations) on a recovery of Cs aromatics of they are as follows:

Conversion,

, Steam or hydrogen, moles/mole of feed None. Hydrogen, 0 21 Steam, 0.221..

Steam, 0.6:1

The eiects of hydrogen and steam on this more comparable basis are shown in graph Il of the accompanying drawing. The considerable improvement and the considerable superiority of steam over hydrogen are manifest. It will be noted that the eilects of steam and hydrogen are more or less lineal with concentrations up to about 0.3 mole per mole of the hydrocarbon feed, after which they tend to level oi. A mole ratio of 6.4:1 did not show any appreciable further improvement. Therefore, while a mole ratio or" 0.2:1 gives a substantial improvement and is applicable, we prefer to employ concentrations between about 0.3:1 and 6:1. When operating at substantially atmospheric pressure, as in the above examples concentrations between about 0.4:1 and about 1:1 are preferred, but when operating at higher pressures higher concentrations of steam or hydrogen give a somewhat further advantage` As stated above, the role of ethylbenzene in the isomerization requires separate consideration. On the one hand, small .amounts in the order of 2%-5% of ethylbenzene are produced during the isomerization of xylenes. On the other hand, ethylbenzene is known to be much more easily cracked than the xylenes [see Ind. Eng. Chem. 37, 1169 (1945)] and under the conditions speciiied it tends to crack at an appreciable rate. Consequently, only very small amounts of xylenes can be produced from ethylbenzene when eiect- Xylenez:` ethylbenzene cracked products when a xylene or a mixture of xylenes is treated by the described process, a small but appreciable amount of the xylene is isomerized to ethylbenzene which then tends to crack. 'I'his can lead to an appreciable loss of xylene. In a preferred embodiment of the present process a certain amount of ethylbenzene is included in the feed. This ethylbenzene may be partly cracked, and if a sucient amount is present it may contribute in a small way to the production of the desired xylene, but this is not important; the main function of the ethylbenzene is to contribute to the xylene yield through its mass action eiect. As shown above, the equilibrium concentration of ethylbenzene is in the order of 8.3 to 13.2 mole afin thcmaterialcxntainingtheemem:

maybereeycled.

Since oleilns areknownto fonn carbonium imisquilereadilywithcatalystsofthetypehere melitwastriedtolxomotethetbn furtin'throughthe addiiion of various small amountsdddinaltwasfounihoweventhat theaddedslightbtheisomermntherthanandalsotended tofoulthecatalystwithus deposits. 'lhell'eaenceofoldinsisevidentlydetrimentu Theahovedcribedtypilofthe invmlimiisinthesehematicprocess sho'ninmmeloftheacoompmying drawin Atylicalt forcarryingout the stepi'silhtratedtically inFigurelIofthedrawing-S. Refen'ingtoFigurellthefeedxylmecntersbyiineiandis llnnpedatthedcsiredfeedratebypump thmhalxehcaterwhereinitisheatedtp thediredreaciim Therequired ammirrtotsteamisiniroducedbylinelandthe mixliirethmpamedtothereactorSwhichis lledwiththecatalyst. 'Iheisomerzedllroduct leaythereactorbylineiandaftercoolingin coolerlil.ispasclltoiiseparatorl Water asalowerlayerwhichis withdrawnlrylinel. Asnallamountofgas iswitbdrawnfmmthetopbylinellandthe xylme mixture is withdrawn by linell.

Weclaimasourinvention: 1. Thepmcesforthe'momerizaton of xylenes which commises contacting a xylene with from 0.2nnleto6molpermoleofxyleneofagas selectedfrtlnthegmupconsistingofsteamand hydmgmatatanperaturebetweenaboutwwl?. and about 1100 F. and at aliquid hourly space veloeiiybetweenaboutlandabout landabove that aifarding substantial disproportionation in the of a. clay-type cracking catalyst.

2. 'Ihepmcsfortheisoxnerization of xylenes w'hkh comprises contacting a xylene with from 0.2moleto6mnlofsteampermoleofxylene atabetweenaboutORand.

ahoutllw'landataliquidhourlyspaeeve-- locilwbetweenalxmtl andabout 16 and above that ama-ding substantial disproportonaton in theofaclay-typecrackingcatalyst.

.3.Aproeesstoclaim2furthercharacta'id in that the xylene treated contains more than the aquilihriiun concentration of ethy Y 4. Apmcesstoclaim2furthercharin that the said clay-type cracking talyst is a synthetic composite of a major amountofslicaandaminoramountof alumina.

5. The process for the isomerization of xylenes which comprkes contacting a xylene with from 0.2moleto6inolesofhydrogenpermole of xylene at a temperature between about 800 F. and about 1100 F. and at a liquid hourly space velocib bet'emaboutl and about 16 and above 6 that affording substantial disproportionatlon in the presence of a clay-type cracking catrlyst.

6. The process for the production of p-xylene from a mixture of isomeric xylenes which comprises contacting a mixture of isomeric i'lenes containing more than the equilibrium concentration of m-xylene at the temperature employed with from 0.2 mole to 6 moles of steam per mole of xylene at a temperature between about 800 F. and about 1100 F. and at a liquid hourly space velocity between 1 and about 1b' and above that affording substantial cracking in the presence of a clay-type cracking catalyst, and recovering,

substantially pure p-xylene from the resulting mixture of isomerized xylenes.

'1. The process for the production of o-xylene from a mixture of three isomeric xylenes approaching the equilibrium composition which comprises'the combination of steps of blending said feed mixture of xylenes with an o-xylenerichgproduct produced as hereinafter specified, separating substantially pure o-xylene from the blend by fractionation, mixing the remaining 'o-xylene-lean mixture with from 0.2 mole to 6 moles of steam per mole of hydrocarbon feed and contacting the mixture at a temperature between'about 800 F. and 1100 F. and at a liquid hourly space velocity between about l and about 16 and above that affording substantial disproportionation with a clay-type cracking catalyst thereby to isomerize said xylenes and produce an o-xylene-rich mixture, and blending said o-xylene-rich mixture with said feed mixture as aforesaid.-

8. The process for the production of substantially pure p-xylene from a mixture of isomeric xylenes approaching the equilibrium composition which comprises the combination of steps of blending said feed mixture of xylenes with an isomerized p-xylene-rich product produced as hereinafter specified, separating a part of .the o-xylene from the blend by fractionation, cooling the remaining o-xylene-lean mixture to crystallize out a portion of the p-xylene, removing the substantially pure p-xylene thus separated, mixing the remaining xylene mixture with said separated o-xylene and with from about 0.2 to about 6 moles of steam per mole of hydrocarbon feed, contacting the resulting mixture at a temperature between about 800 F. and 1100 F.

at a liquid hourly space velocity between about 1 and 'about 16 and abovevthat affording substantial disproportionation with a clay-type cracking catalyst. thereby to isomerize said xylenes and produce a p-xylene-rich mixture, and blending said p-xylene-rich mixture with said feed mixture as aforesaid.

9. .A process for the production of o-xylene from a mixture of isomeric xylenes rich in m-xyiene which comprises the combination of steps of blending said 'feedwith an o-xylene-rich isomerized productcontaining more thanthe equilibrium concentration of ethylbenzene, separating a part of the o-xylene from the blend by fractionation, mixing the remaining o-xylenelean mixture containing said ethylbenzene with from about 0.2 to about 6 moles of steam per mole of hydrocarbon, contacting the mixture at a temperature between about 800 F. and 1100 F. and at a liquid hourly space velocity between about 1 and about 16 and above that affording substantial side reactions with a clay-type cracking catalyst thereby to isomerize said xylenes and produce an o-xylene-rich mixture containing said s,sa4,sss 7 emylbenzene. and blending said o-xylene rich mixture with said feed as aforesaid.

The following references are of record 1n the le of this patent:

8 UNITED STATES PATENTS Name Date Ldov Feb. 1, 1944 .Welsy` July 10, 1945 Reeves July 9, 1946 Passino et al Aug. 12, 1947 

1. THE PROCESS FOR THE ISOMERIZATION OF XYLENES WHICH COMPRISES CONTACTING A XYLENE WITH FROM 0.2 MOLE TO 6 MOLES PER MOLE OF XYLENE OF A GAS SELECTED FROM THE GROUP CONSISTING OF STEAM AND HYDROGEN AT A TEMPERATURE BETWEEN ABOUT 800* F. AND ABOUT 1100* F. AND AT A LIQUID HOURLY SPACE VELOCITY BETWEEN ABOUT 1 AND ABOUT 16 AND ABOVE THAT AFFORDING SUBSTANTIAL DISPROPORTIONATION IN THE PRESENCE OF A CLAY-TYPE CRACKING CATALYST. 